<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>取消请求</title>
</head>
<body>
    <button>发送请求</button>
    <script>
        const btns = document.querySelectorAll('button');
        let x = null;
        // 增加一个标识变量来判断是否正在发送请求
        let isSending = false;

        btns[0].onclick = function() {
            // 判断标识变量
            if(isSending) x.abort();
            x = new XMLHttpRequest();
            isSending = true;
            x.open('GET','http://127.0.0.1:8000/delay');
            x.send();
            x.onreadystatechange = function() {
                if(x.readyState === 4) {
                    isSending = false;
                }
            }
        }
    </script>
</body>
</html>